Hybrid vehicles may employ Belt Alternator Starter (BAS) technology to gain energy efficiency. Coming to a stop at a traffic light or during an extended idle, the engine is turned off to achieve enhanced fuel economy. As the brake pedal is released, an electric motor/generator unit instantaneously restarts the engine, typically in less than half of a second time, making the auto start system essentially transparent to the driver. This is referred to as the Stop-Start strategy for enhancing fuel economy. A BAS vehicle can provide 15-20% fuel economy gain in the city and an overall fuel economy increase of 4˜7%. For a baseline gasoline vehicle with 30 mpg fuel economy, this is equivalent to an increase of 1.2˜2.1 mpg of Fuel economy improvement.
While the Stop-Start strategy improves fuel economy, it may compromise passenger comfort. Stopping the engine disables the belt-driven A/C system, resulting in interruption of cooling for the passenger compartment. Currently, vehicle OEM's currently rely on the thermal inertia of the air conditioning evaporator to provide some residual cooling during the period when the engine is stopped. The evaporator residual cooling time is typically limited to 25 seconds or less before the discharge temperature of the evaporator rises above a level that no longer provides the desired cooling. When the evaporator is warmed up to a specified air discharge temperature, the engine is restarted to drive the A/C system to provide cooling. This periodic restart under idle conditions undermines average fuel economy that can be achieved by the Hybrid vehicles.